This is seriously cold weather, folks.
Pack a bit extra of everything so you can change out of wet stuff into dry at least once daily. An extra pair of dry socks, dry shoes, and dry gloves will go a long way—as will a warm knit hat exclusively for sleeping (in addition to one to wear during the day). Also don’t forget snow or rain pants. You will be kneeling and sitting in the snow a lot.
Finally: Don’t forget sunscreen. Kids often get more burned in winter ’cause they don’t bother, yet conditions are worse, what with sunlight reflecting off all the snow.